Health monitoring of structures which are used for long-term and especially in harsh conditions has become increasingly important. Perfect functionality and mechanical integrity are fundamental requirements for structural parts of aircraft that over time can suffer from defects like fractures and fatigue microcracks. As a consequence, if such defects are over-looked, the efficiency of material is strongly compromised and may result in dangerous accidents. Therefore, structural health monitoring (SHM) of fundamental parts of aircrafts (i.e. fuselage, wings, cockpit) that are constantly exposed to threatening environmental effects such as temperature changes, impact by birds or hailstones, lightning strikes, is crucial in order to ensure the safety, serviceability and reliability of the vehicles. At present, the monitoring of such structures, mainly based on non-destructive testing (NDT), such as a X-ray and ultrasonic inspection, acceleration-based modal testing, is time consuming and expensive specially for the structures to be monitored that must be disassembled and transported to testing facilities. For years, the integration of functional components as strain sensors within composite materials has been a challenge in material science as alternative to classical technique. The goal is the development of an embedded strain sensing system in a composite structure having both functions of structural damage identification and sensor self-diagnosis. In situ stress and strain detection, together with structural health monitoring, would provide improved durability and safety of composite structures. An attempt in this direction is disclosed by U.S. Pat. No. 8,384,398 B2 that provides a structural health monitoring in order to localize crack damage in the monitoring of civil structures and infrastructures (such as buildings, bridges, tunnels) by means of one or more capacitive sensor to be applied as sensing skin on the area of interest to be monitored. In the field of nanocomposites, patent CA 2 570 117 C provides a sensing system formed from a conductive ink containing carbon nanofibers and a polymeric resin that must be applied directly to the structure to be monitored in the form of a grid pattern. Damage to the structure may be evaluated in terms of resistance values detected from the sensor in agreement with the classical real-time sensing approach based on piezoresistive material placed on the monitored structure surface and on the detection of the strain-induced electric charge or current, that however suffers from high cost, small sensing area and low durability due to the poor adhesive properties between the parts not able to ensure that the sensor does not peel over the time.
US 2013/0312535 discloses a method for monitoring the health of a structure by applying thereto a sprayable paint formulation, whose variations of electrical resistance under mechanical stress are measured. Hence, the structure to be monitored and the coating paint on which measurements are effected are neatly distinguished.
An object of the present invention is therefore providing an enhanced method of sensing and monitoring in composite materials dangerous stresses, which might even bring about catastrophic failures.